偏極ターゲット用He 3冷凍機

抄録

In order to measure the polarized target asymmetry parameters in the reactions of the pion photoproduction, i.e. γ+p^↑↓→π⁺+n and π°+p, we constructed the polarized proton target which included the helium 3 refrigerator system. This is a report on some details of the cryostat, the pumping system and the purifier and suroundings under the long-run operating condition. The cryostat is a so-called “Roubeau type” one which has a characteristic of easy handling by separating the helium 3 system from the helium 4 one. The Roots pump for evacuation of the helium 3 had a effective pumping speed of 200 1/sec and kept the vapour pressure 0.12 torr. The purifier consists of spiral tubing and 1.51 charcoal in the temperature of the liquid nitrogen. Our cryostat needed only 3 hours for cooling down to 0.5K by changing the original calibrated leak to needle valve and adding the by-pass line from the condensor to the cavity. This shortening of the time for precooling was a key of the success of the experiment, because the target must be changed when the polarization decreases due to the radiation damage, which is remarkable in the experiment of photon or electron beam. The flow rate of the helium 3 was 0.6mmole/sec and had the cooling power of 20mW at the cavity. The average consumption rate of the liquid helium 4 from the separator and the evaporator were 2.61/h and 0.61/h, respectively. The small frozen sphere of the butanol doped with the paramagnetic impurity named porphyrexide was used as targets set in the FEP box. The target was changed every one week before the appearance of the depolarization. The average polarization was 65% throughout the experiment of 4 months.